The present invention generally relates to the preparation of semiconductor material substrates, especially silicon wafers, which are used in the manufacture of electronic components. More particularly, the present invention relates to a process for the treatment of silicon wafers which enables the wafers, during the heat treatment cycles of essentially any arbitrary electronic device manufacturing process to form an ideal, non-uniform depth distribution of oxygen precipitates.
Single crystal silicon, which is the starting material for most processes for the fabrication of semiconductor electronic components, is commonly prepared with the so-called Czochralski process wherein a single seed crystal is immersed into molten silicon and then grown by slow extraction. As molten silicon is contained in a quartz crucible, it is contaminated with various impurities, among which is mainly oxygen. At the temperature of the silicon molten mass, oxygen comes into the crystal lattice until it reaches a concentration determined by the solubility of oxygen in silicon at the temperature of the molten mass and by the actual segregation coefficient of oxygen in solidified silicon. Such concentrations are greater than the solubility of oxygen in solid silicon at the temperatures typical for the processes for the fabrication of electronic devices. As the crystal grows from the molten mass and cools, therefore, the solubility of oxygen in it decreases rapidly, whereby in the resulting slices or wafers, oxygen is present in supersaturated concentrations.
Thermal treatment cycles which are typically employed in the fabrication of electronic devices can cause the precipitation of oxygen in silicon wafers which are supersaturated in oxygen. Depending upon their location in the wafer, the precipitates can be harmful or beneficial. Oxygen precipitates located in the active device region of the wafer can impair the operation of the device. Oxygen precipitates located in the bulk of the wafer, however, are capable of trapping undesired metal impurities that may come into contact with the wafer. The use of oxygen precipitates located in the bulk of the wafer to trap metals is commonly referred to as internal or intrinsic gettering (xe2x80x9cIGxe2x80x9d).
Historically, electronic device fabrication processes included a series of steps which were designed to produce silicon having a zone or region near the surface of the wafer which is free of oxygen precipitates (commonly referred to as a xe2x80x9cdenuded zonexe2x80x9d or a xe2x80x9cprecipitate free zonexe2x80x9d) with the balance of the wafer, i.e., the wafer bulk, containing a sufficient number of oxygen precipitates for IG purposes. Denuded zones can be formed, for example, in a high-low-high thermal sequence such as (a) oxygen out-diffusion heat treatment at a high temperature ( greater than 1100xc2x0 C.) in an inert ambient for a period of at least about 4 hours, (b) oxygen precipitate nuclei formation at a low temperature (600-750xc2x0 C.), and (c) growth of oxygen (SiO2) precipitates at a high temperature (1000-1150xc2x0 C.). See, e.g., F. Shimura, Semiconductor Silicon Crystal Technology, Academic Press, Inc., San Diego Calif. (1989) at pages 361-367 and the references cited therein.
More recently, however, advanced electronic device manufacturing processes such as DRAM manufacturing processes have begun to minimize the use of high temperature process steps. Although some of these processes retain enough of the high temperature process steps to produce a denuded zone and sufficient density of bulk precipitates, the tolerances on the material are too tight to render it a commercially viable product. Other current highly advanced electronic device manufacturing processes contain no out-diffusion steps at all. Because of the problems associated with oxygen precipitates in the active device region, therefore, these electronic device fabricators must use silicon wafers which are incapable of forming oxygen precipitates anywhere in the wafer under their process conditions. As a result, all IG potential is lost.
Among the objects of the invention, therefore, is the provision of a single crystal silicon wafer which, during the heat treatment cycles of essentially any electronic device manufacturing process, will form an ideal, non-uniform depth distribution of oxygen precipitates; the provision of such a wafer which will optimally and reproducibly form a denuded zone of sufficient depth and a sufficient density of oxygen precipitates in the wafer bulk; the provision of such wafer in which the formation of the denuded zone and the formation of the oxygen precipitates in the wafer bulk is not dependent upon difference in oxygen concentration in these regions of the wafer; the provision of such a wafer in which the thickness of the resulting denuded zone is essentially independent of the details of the integrated circuit manufacturing process sequence; the provision of such a wafer in which the formation of the denuded zoned and the formation of the oxygen precipitates in the wafer bulk is not influenced by the thermal history and the oxygen concentration of the Czochralski-grown single crystal silicon ingot from which the silicon wafer is sliced; the provision of a process in which the formation of the denuded zone does not depend upon the out-diffusion of oxygen; and the provision of a process in which the silicon is doped with nitrogen and/or carbon at a sufficient concentration to stabilize oxygen precipitate nucleation centers such that they can withstand subsequent rapid thermal processing without preventing the formation of the denuded zone.
Briefly, therefore, the present invention is directed to a single crystal silicon wafer having two major, generally parallel surfaces, one of which is the front surface of the wafer and the other of which is the back surface of the wafer, a central plane between the front and back surfaces, and a circumferential edge joining the front and back surfaces. The wafer also comprises a dopant selected from a group consisting of nitrogen and carbon, the concentration of the dopant being sufficient to promote the formation of stabilized oxygen precipitate nucleation centers as the wafer is cooled from a first temperature, T1, to a second temperature, T2, at a rate, R. The stabilized oxygen precipitation nucleation centers being incapable of are dissolved at a temperature less than about 1150xc2x0 C. but capable of being dissolved at a temperature between about 1150xc2x0 C. and about 1300xc2x0 C., T1 is between about 1150xc2x0 C. and about 1300xc2x0 C., T2 is a temperature at which crystal lattice vacancies are relatively immobile in silicon, and R is at least about 5xc2x0 C. per second. Still further, the wafer comprises a surface layer which comprises the region of the wafer between the front surface and a distance, D, measured from the front surface and toward the central plane, wherein the surface layer is free of stabilized oxygen precipitate nucleation centers. Also, the wafer comprises a bulk layer which comprises a second region of the wafer between the central plane and the surface layer, wherein the bulk layer comprises stabilized oxygen precipitate nucleation centers.
The present invention is also directed to a single crystal silicon wafer having two major, generally parallel surfaces, one of which is the front surface of the wafer and the other of which is the back surface of the wafer, a central plane between the front and back surfaces, and a circumferential edge joining the front and back surfaces. The wafer comprises a dopant selected from a group consisting of nitrogen and carbon, when nitrogen is the dopant, the the concentration of the nitrogen is between about 1xc3x971012 and about 5xc3x971014 atoms/cm3, and when carbon is the dopant, the concentration of carbon is between about 1xc3x971016 and about 4xc3x971017 atoms/cm3. The wafer also comprises a surface layer that comprises the region of the wafer between the front surface and a distance, D, measured from the front surface and toward the central plane, wherein the surface layer is free of stabilized oxygen precipitate nucleation centers. Additionally, the wafer comprises a bulk layer that comprises a second region of the wafer between the central plane and the surface layer, wherein the bulk layer comprises stabilized oxygen precipitate nucleation centers.
In yet another embodiment, the present invention is directed to a silicon on insulator structure comprising single crystal handle wafer, a single crystal silicon device layer, and an insulating layer between the handle wafer and the device layer. The single crystal silicon handle wafer comprises two major, generally parallel surfaces, one of which is the front surface of the wafer and the other of which is the back surface of the wafer, a central plane between the front and back surfaces, and a circumferential edge joining the front and back surfaces. The handle wafer also comprises a dopant selected from a group consisting of nitrogen and carbon, the concentration of the dopant being sufficient to promote the formation of stabilized oxygen precipitate nucleation centers as the wafer is cooled from a first temperature, T1, to a second temperature, T2, at a rate, R. The stabilized oxygen precipitation nucleation centers are incapable of being dissolved at a temperature less than about 1150xc2x0 C. but capable of being dissolved at a temperature between about 1150xc2x0 C. and about 1300xc2x0 C., T1 is between about 1150xc2x0 C. and about 1300xc2x0 C., T2 is a temperature at which crystal lattice vacancies are relatively immobile in silicon, and R is at least about 5xc2x0 C. per second. The handle wafer further comprises a surface layer which comprises the region of the wafer between the front surface and a distance, D, measured from the front surface and toward the central plane, wherein the surface layer is free of stabilized oxygen precipitate nucleation centers. Also, the handle wafer comprises a bulk layer which comprises a second region of the wafer between the central plane and the surface layer, wherein the bulk layer comprises stabilized oxygen precipitate nucleation centers.
The present invention is further directed to a process for the preparation of a single crystal silicon wafer having a controlled oxygen precipitation behavior. The process comprises selecting a wafer sliced from a single crystal silicon ingot grown by the Czochralski method and comprises a front surface, a back surface, a central plane between the front and back surfaces, a front surface layer which comprises the region of the wafer between the front surface and a distance, D, measured from the front surface and toward the central plane, a bulk layer which comprises the region of the wafer between the central plane and the front surface layer. The wafer also comprises a dopant selected from a group consisting of nitrogen and carbon, the concentration of the dopant being sufficient to promote the formation of stabilized oxygen precipitate nucleation centers as the wafer is cooled from a first temperature, T1, to a second temperature, T2, at a rate, R. The stabilized oxygen precipitation nucleation centers are incapable of being dissolved at a temperature less than about 1150xc2x0 C. but capable of being dissolved at a temperature between about 1150xc2x0 C. and about 1300xc2x0 C., T1 is between about 1150xc2x0 C. and about 1300xc2x0 C., T2 is a temperature at which crystal lattice vacancies are relatively immobile in silicon, and R is at least about 5xc2x0 C. per second. The process comprises heating the wafer to a temperature of at least about 1150xc2x0 C. to form crystal lattice vacancies in the front surface and bulk layers and cooling the heated wafer at a rate to form a vacancy concentration profile in the wafer wherein the peak density of vacancies is in the bulk layer with the concentration generally decreasing from the location of the peak density in the direction of the front surface of the wafer and the difference in the concentration of vacancies in the front surface and bulk layers is such that stabilized oxygen precipitate nucleation centers do not form in the front surface layer and stabilized oxygen precipitate nucleation centers form in the bulk layer. Also, the process comprises forming stabilized oxygen precipitate nucleation centers in the bulk layer as the wafer is being cooled with the concentration of the stabilized oxygen precipitate nucleation centers in the bulk layer being primarily dependant upon the concentration of vacancies.
In yet another embodiment, the present invention is directed to a process for the preparation of a single crystal silicon wafer having a controlled oxygen precipitation behavior. The process comprises selecting a wafer is sliced from a single crystal silicon ingot grown by the Czochralski method comprising a front surface, a back surface, a central plane between the front and back surfaces, a front surface layer which comprises the region of the wafer between the front surface and a distance, D, measured from the front surface and toward the central plane, a bulk layer which comprises the region of the wafer between the central plane and the front surface layer. The wafer also comprises a dopant selected from the group consisting of nitrogen and carbon, when nitrogen is the dopant, the concentration of the nitrogen is between about 1xc3x971012 and about 5xc3x971014 atoms/cm3, and when carbon is the dopant, the concentration of carbon is between about 1xc3x971016 and about 4xc3x971017 atoms/cm3. The process also comprises subjecting the wafer to a heat-treatment to form crystal lattice vacancies in the front surface and bulk layers. The heated-treated wafer is cooled at a rate that produces a vacancy concentration profile in the wafer wherein the peak density of vacancies is in the bulk layer with the concentration generally decreasing from the location of the peak density in the direction of the front surface of the wafer and the difference in the concentration of vacancies in the front surface and bulk layers is such that stabilized oxygen precipitate nucleation centers do not form in the front surface layer and stabilized oxygen precipitate nucleation centers form in the bulk layer. Additionally, as the heat-treated wafer is cooled, stabilized oxygen precipitate nucleation centers are formed in the bulk layer with the concentration of the stabilized oxygen precipitate nucleation centers in the bulk layer being primarily dependant upon the concentration of vacancies.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.